1. Field of the Invention
This invention relates to semiconductor devices each incorporating a unit lead frame made as by performing a mold array process (MAP). The invention is also directed to a method of making a semiconductor device.
2. Background Art
It is known to make semiconductor devices by bonding semiconductor chips to a lead frame and applying a sealing resin over the chip at one side of the unit lead frame. In recent years, the desire to miniaturize, and provide higher density, semiconductor devices has caused the proliferation of, among other semiconductor devices, those known as SON (small outline non-leaded package) and QFN (quad flat non-leaded package) semiconductor devices.
Reference is made to FIGS. 21-23 herein wherein this type of semiconductor device is shown at 10. The semiconductor device 10 has leads 12 which do not project at a peripheral edge 14 of the semiconductor device 10 beyond a resin layer 16. The leads 12 are exposed in this design at a back side 18 of the semiconductor device 10.
It is known to manufacture semiconductor devices with substrates made from PCB, tape, and the like, practicing a manufacturing method known as a MAP process (mold array process). With this process, a sheet 20 is conventionally formed into a lead frame 22 with a base rail 24 connected to and surrounding a matrix of unit lead frames 26, identified as A-I. Drive openings 27 facilitate manipulation of the base rail 24. As described in greater detail below, the components of the semiconductor device 10 are built up upon the lead frame 22, which is ultimately cut to separate the individual semiconductor devices 10 (in this case nine (9) in number) from each other and the base rail 24. This process has been practiced to make the aforementioned SON and QFN semiconductor devices, as well as other types of semiconductor devices.
More particularly, in carrying out the MAP process to produce the semiconductor devices 10, the lead frame 22 is formed as shown in FIG. 21. Each unit lead frame 26 has a similar construction and, as shown for exemplary lead frame 26 at A, consists of a rectangular support 28 with four leads 12 at each of four outer edge portions 30, 32, 34, 36 cooperatively defining the peripheral outer edge 38 of the support 28. A semiconductor chip 40 is bonded to the support 28 using an adhesive or an adhesive tape 42. Electrodes 44 on the semiconductor chip 40 are electrically connected to the leads 12 through conductive wires 46.
The supports 28, leads 12, and base rail 24 are maintained in a desired relative position by a tie bar network 48. The tie bar network 48 consists of a plurality of elongate tie bars, including support tie bars 50, projecting in a diagonal direction each from a corner of the support, and peripheral tie bars 52 (numbered for the unit lead frame 26 at E), which extend around the perimeter of the individual unit lead frames 26 and interconnect with each other, the leads 12, the support tie bars 50, and the base rail 24.
Once the semiconductor chips 40 are applied and electrically connected to the leads 12, the resin layer 16 is applied in sealing fashion. The resin layer 16 is applied so as to simultaneously seal all of the unit lead frames 26 (A-I) continuously over the matrix of unit lead frames 26 within the square line 54.
The individual semiconductor device 10 are separated and completed by strategically cutting a lead frame assembly, consisting of the lead frame 22 with the semiconductor chips 40, conductive wires 46, and resin layer 16 thereon, along lines indicated by the arrows A, and lines orthogonal thereto, as indicated by the arrows B. This effects separation of the semiconductor devices 10 from each other and the base rail 24. This cutting may be effected using a saw with a width at least as large as the width W of the peripheral tie bars 52.
The MAP process is desirable from the standpoint that the resin application can be carried out simultaneously for all of the semiconductor devices 10. A single die can be used to facilitate the resin application. This process lends itself to being carried out efficiently and economically.
However, the MAP process described above, using the conventional structure shown in FIGS. 21-23, has a number of inherent problems. One problem results from the difference in the hardness of the material defining the lead frame 22 and the resin layer 16. As the cutting blade cuts through the lead frame assembly, the resistance to cutting is different by reason of the different hardness of the two materials. As a result of this, there may be different deformation resulting from the cutting operation at different locations around the periphery of the semiconductor device 10. This condition may cause a separation between the resin layer 16 and parts of the lead frame 22 at their interface. As just one example, this separation phenomenon is shown at 56 in FIG. 23 where the resin layer 16 and lead frame 22 are bonded prior to performance of the cutting operation. Generally, this condition occurs in a downstream direction on a part of the lead frame 22 with respect to the cutting direction, as indicated by the arrows C in FIG. 23.
An additional problem is that the cutting may form burrs 58 at the corners 59 to which the support tie bars 50 project. These burrs 58 likewise tend to form on the four tie bars 50 in a downstream region with respect to the cutting direction, as indicated by the arrow C. These burrs 58 not only affect the appearance and dimensions of the semiconductor devices 10, but may also compromise the quality of an electrical connection to the semiconductor device 10. To alleviate this problem, a separate deburring step may be required. This potentially complicates the manufacturing process and increases attendant costs.
In one form, the invention is directed to a lead frame for a semiconductor device. The semiconductor device has a sheet with oppositely facing sides and a thickness between the oppositely facing sides. The sheet has first and second unit lead frames. Each unit lead frame has a support for a semiconductor chip and at least one lead space from the support. The sheet has a tie bar network which connects a) the support to the at least one lead on each of the first and second lead frames and b) the first and second lead frames, each to the other. The sheet has a dividing line along which the sheet can be cut to separate the first and second lead frames from each other. The tie bar network consists of at least one tie bar extending along a substantial length of the dividing line. The support has a first thickness between the oppositely facing sides of the sheet. The at least one tie bar has a second thickness between the oppositely facing sides of the sheet over a substantial length of the dividing line that is less than the first thickness.
In one form, the base on the first unit lead frame has a polygonal shape with an outer edge defined by a plurality of straight edge portions. The dividing line is substantially straight and has a length and is spaced from, and extends substantially parallel to, one of the straight edge portions. The at least one tie bar has a thickness between the oppositely facing sides of the sheet that is less than the first thickness over substantially the entire length of the dividing line.
In one form, the first unit lead frame has a corner and the tie bar network consists of a support tie bar assembly including at least one support tie bar that extends from the support on the first unit lead frame towards the corner.
The second difference in the lead frame 142 is at the leads 66xe2x80x2. Each lead 66xe2x80x2 is formed with an undercut 160 along a substantial length thereof, which undercut resides fully within, and is spaced from, side edges bounding the width of the lead 66xe2x80x2.
In one form, the support tie bar assembly has a length and a thickness between the oppositely facing sides of the sheet that is less than the first thickness over at least a portion of the length of the support tie bar assembly.
The at least one support tie bar may have a discrete opening therethrough.
In one form, the discrete opening is fully surrounded by the at least one support tie bar.
The discrete opening may be an elongate opening.
At least one lead on at least one of the unit lead frames may have an undercut formed therein.
In one form, the first and second unit lead frames each have a rectangular shape with a peripheral edge defined by first, second, third, and fourth peripheral edge portions. The support on the first unit lead frame has a rectangular shape defined by first, second, third, and fourth outer edges. The first, second, third and fourth peripheral edge portions are substantially parallel to the first, second, third and fourth outer edges. There are a plurality of leads between the first peripheral edge portion and the first outer edge, the second peripheral edge portion and the second outer edge, the third peripheral edge portion and the third outer edge, and the fourth peripheral edge portion and the fourth outer edge.
In one form, the sheet has of a border rail and the tie bar network connects the first unit lead frame to the border rail. The first unit lead frame has a peripheral edge connected to the border rail through the tie bar network.
The sheet may have a second dividing line along the peripheral edge along which the sheet can be cut to separate the first unit lead frame from at least a part of the border rail. The tie bar network may have at least a second tie bar extending along a substantial length of the second dividing line. The at least second tie bar has a thickness between the oppositely facing sides of the sheet over a substantial length of the second dividing line that is less than the first thickness.
In one form, the support on the first unit lead frame is on a first side of the second dividing line and the at least part of the border rail is on a second side of the second dividing line. The at least one lead on the first unit lead frame and tie bar network are sufficiently symmetrical at the first and second sides of the second dividing line that resistance to cutting along the second dividing line at the first and second sides is substantially the same.
In one form, the support tie bar assembly extends to the corner at which the first and second peripheral tie bars meet and the sheet at the corner at which the first and second peripheral tie bars meet has a thickness that is less than the first thickness.
In one form, at least one of the oppositely facing sides of the sheet is formed to produce the second thickness.
The at least one of the oppositely facing sides may be formed by one of etching and compression.
In one form, the first unit lead frame has a peripheral edge defined by a plurality of peripheral edge portions. The tie bar network and the at least one lead extend continuously around the first unit lead frame so as to connect the first unit lead frame to a) the second unit lead frame, b) the border rail, and c) at least a third unit lead frame. At least a portion of the tie bar network has a thickness less than the first thickness extending substantially fully around the peripheral edge of the first unit lead frame.
In one form, the at least one lead has a portion with a thickness that is less than the first thickness.
In one form, the tie bar network has a thickness less than the first thickness extending continuously fully around the first unit lead frame.
The invention is also directed to a semiconductor assembly consisting of a lead frame, as previously described, a first semiconductor chip applied to the support on the first unit lead frame, a first conductive element electrically connecting the first semiconductor chip to the at least one lead on the first unit lead frame, a second semiconductor chip applied to the support on the second unit lead frame, a second conductive element electrically connecting the second semiconductor chip to the at least one lead on the second unit lead frame, and a resin layer applied over one of the oppositely facing sides of the sheet so as to be applied to the first and second semiconductor chips and the first and second conductive elements.
The invention is also directed to a method of forming a semiconductor device including the steps of: providing a semiconductor assembly as described above, and forming a first semiconductor device by cutting through the resin layer and the lead frame around the first unit lead frame including along the dividing line.
The invention is further directed to a semiconductor device consisting of a sheet portion defining a unit lead frame with oppositely facing sides and a thickness between the oppositely facing sides of the sheet portion. The unit lead frame has a support with a first thickness between the oppositely facing sides of the sheet portion. The unit lead frame has at least one lead and a tie bar network made up of a plurality of elongate tie bars, each with a length. The tie bar network connects the support to the at least one lead. A semiconductor chip is provided on the support. A conductive element electrically connects the semiconductor chip to the at least one lead. A resin layer is applied over the semiconductor chip, the conductive element, and at least a part of one of the oppositely facing sides of the sheet portion. The semiconductor device has a peripheral edge made up of a plurality of straight edge portions defining a polygonal shape. A plurality of the peripheral edge portions are formed by cutting to expose a part of the tie bar network. A first plurality of the elongate tie bars each have a substantial length that has a thickness between the oppositely facing sides of the sheet that is less than the first thickness.